U.S. patent application number 13/038990 was filed with the patent office on 2011-07-14 for method and device for stabilizing the state of polarization of a polarization multiplexed optical radiation.
This patent application is currently assigned to PGT Photonics S.p.A.. Invention is credited to Pierpaolo Boffi, Lucia Marazzi, Paolo Martelli, Mario Martinelli, Aldo Righetti, Rocco Siano.
Application Number | 20110170870 13/038990 |
Document ID | / |
Family ID | 34959122 |
Filed Date | 2011-07-14 |
United States Patent
Application |
20110170870 |
Kind Code |
A1 |
Boffi; Pierpaolo ; et
al. |
July 14, 2011 |
Method and Device for Stabilizing the State of Polarization of a
Polarization Multiplexed Optical Radiation
Abstract
A device and method for stabilizing the state of polarization of
polarization multiplexed optical radiation including an identified
channel is disclosed. The device and method comprise providing to
the polarization multiplexed optical radiation a first controllable
polarization transformation to generate a first transformed optical
radiation; detecting a first state of polarization of a first
polarized portion with respect to a first polarization parameter;
controlling the first controllable polarization transformation so
that the first polarization parameter has a predetermined value
independent of a polarization state of the polarization multiplexed
optical radiation; providing to the first transformed optical
radiation a second controllable polarization transformation to
generate a second transformed optical radiation; detecting a second
state of polarization of a second polarized portion; and
controlling the second controllable polarization transformation so
that the second state of polarization has a predefined value.
Inventors: |
Boffi; Pierpaolo; (Voghera
(PV), IT) ; Marazzi; Lucia; (Pavia (PV), IT) ;
Martelli; Paolo; (Milan, IT) ; Martinelli; Mario;
(San Donato Milanese (Mi), IT) ; Righetti; Aldo;
(Milano (MI), IT) ; Siano; Rocco; (Milano (MI),
IT) |
Assignee: |
PGT Photonics S.p.A.
|
Family ID: |
34959122 |
Appl. No.: |
13/038990 |
Filed: |
March 2, 2011 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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11665466 |
Apr 16, 2007 |
7917031 |
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PCT/EP2004/011957 |
Oct 22, 2004 |
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13038990 |
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Current U.S.
Class: |
398/65 |
Current CPC
Class: |
H04J 14/06 20130101;
G02F 1/0136 20130101; H04J 14/02 20130101 |
Class at
Publication: |
398/65 |
International
Class: |
H04J 14/06 20060101
H04J014/06 |
Claims
1.-24. (canceled)
25. A method for stabilizing the state of polarization of
polarization multiplexed optical radiation, said polarization
multiplexed optical radiation comprising an identified channel
which is provided with a pilot signal, comprising: (a) providing to
the polarization multiplexed optical radiation a first controllable
polarization transformation to generate a first transformed optical
radiation; (b) detecting a first state of polarization of a first
polarized portion of the identified channel of the first
transformed optical radiation with respect to a first polarization
parameter of the first transformed optical radiation; (c)
controlling, responsively to said first state of polarization, the
first controllable polarization transformation so that the first
polarization parameter of the first transformed optical radiation
has a predetermined value independent of a polarization state of
the polarization multiplexed optical radiation; (d) providing to
the first transformed optical radiation a second controllable
polarization transformation to generate a second transformed
optical radiation; (e) detecting a second state of polarization of
a second polarized portion of the identified channel of the second
transformed optical radiation; and (f) controlling, responsively to
said second state of polarization, the second controllable
polarization transformation so that the second state of
polarization has a predefined value.
26. The method according to claim 25, wherein said first
polarization parameter includes an ellipticity of the first
transformed optical radiation.
27. The method according to claim 25, wherein said second polarized
portion of said identified channel of the second transformed
optical radiation has a state of polarization parallel or
perpendicular to said first state of polarization.
28. The method according to claim 25, wherein step (b) comprises
measuring a modulation amplitude of said pilot signal.
29. The method according to claim 28, wherein step (b) comprises
extracting a power fraction from the first transformed optical
radiation, polarizing said power fraction to generate a polarized
power fraction, detecting said polarized power fraction and
pass-band filtering said detected polarized power fraction to
obtain said modulation amplitude.
30. The method according to claim 25, wherein step (e) comprises
measuring modulation amplitude of said pilot signal.
31. The method according to claim 30, wherein step (e) comprises
extracting a power fraction from the second transformed optical
radiation, polarizing said power fraction to generate a polarized
power fraction, detecting said polarized power fraction and
pass-band filtering said detected polarized power fraction to
obtain said modulation amplitude.
32. The method according to claim 25, further comprising detecting
a third state of polarization with respect to the first
polarization parameter of a third polarized portion of said
identified channel of the first transformed optical radiation.
33. The method according to claim 32, wherein said third polarized
portion of said identified channel of the first transformed optical
radiation has the first polarization parameter orthogonal to the
first polarization parameter of said first polarized portion.
34. The method according to claim 25, wherein the first
controllable polarization transformation is endlessly varying.
35. The method according to claim 25, wherein the second
controllable polarization transformation is endlessly varying.
36. A method of demultiplexing polarization multiplexed optical
radiation, comprising the method of stabilizing the state of
polarization of polarization multiplexed optical radiation, wherein
said polarization multiplexed optical radiation comprises an
identified channel which is provided with a pilot signal, according
to claim 25, and further comprising separating the identified
channel in the second transformed optical radiation from a further
channel orthogonally polarized to the identified channel.
37. A method of transmitting a polarization multiplexed optical
signal comprising: providing a pilot signal to an optical channel
to generate an identified channel; polarization multiplexing the
identified channel with a further channel at a first location to
generate polarization multiplexed optical radiation; propagating
said polarization multiplexed optical radiation at a second
location different from the first location; stabilizing the state
of polarization of the polarization multiplexed optical radiation
at the second location according to the method of claim 25, to
generate a polarization stabilized optical radiation; separating
the identified channel of the polarization stabilized optical
radiation from the further channel; and detecting at least one of
said identified and further channel.
38. A device for stabilizing the state of polarization of
polarization multiplexed optical radiation, said polarization
multiplexed optical radiation comprising an identified channel
which is provided with a pilot signal, comprising: a first
polarization transformer comprising a first birefringent element
operable to provide a first variable polarization transformation to
the polarization multiplexed optical radiation; a first monitoring
system responsive to said pilot signal and capable of detecting a
first state of polarization of a first polarized portion of the
identified channel with respect to a first polarization parameter
downstream of the first polarization transformer; a controller
capable of controlling responsively to the first state of
polarization of said first polarized portion, said first variable
polarization transformation so as to maintain the first
polarization parameter of the identified channel downstream of the
first polarization transformer at a predefined value independent of
a polarization state of the polarization multiplexed optical
radiation; a second polarization transformer positioned downstream
of the first polarization transformer and comprising a second
birefringent element operable to provide a second variable
polarization transformation to the polarization multiplexed optical
radiation; and a second monitoring system responsive to said pilot
signal and capable of detecting a second state of polarization of a
second polarized portion of the identified channel downstream of
the second polarization transformer, the controller being further
capable of controlling, responsively to the second state of
polarization of said second polarized portion, said second variable
polarization transformation so as to maintain the second state of
polarization at a predefined value.
39. The polarization stabilizing device according to claim 38,
wherein the first monitoring system is further capable of detecting
a third state of polarization with respect to the first
polarization parameter of a further polarized portion of the
identified channel downstream of the first polarization
transformer, wherein said further polarized portion is orthogonal
to the first polarized portion.
40. The polarization stabilizing device according to claim 38,
wherein the first polarization transformer further comprises a
third birefringent element operable to provide a third variable
polarization transformation to the polarization multiplexed optical
radiation.
41. The polarization stabilizing device according to claim 40,
wherein the controller is configured to switch the third variable
polarization transformation between first and second values when
the first variable polarization transformation reaches a predefined
threshold value.
42. The polarization stabilizing device according to claim 40,
wherein each of the first birefringent element and the third
birefringent element comprises a respective variable rotator and
the first polarization transformer further comprises a quarter-wave
plate optically interposed between the first and the third
birefringent element.
43. The polarization stabilizing device according to claim 40,
wherein the second polarization transformer further comprises a
fourth birefringent element operable to provide a fourth variable
polarization transformation to the polarization multiplexed optical
radiation.
44. The polarization stabilizing device according to 43, wherein
the controller is configured to switch the fourth variable
polarization transformation between third and fourth values when
the second variable polarization transformation reaches a
predefined threshold value.
45. The polarization stabilizing device according to claim 38,
wherein the first monitoring system is configured to measure a
modulation amplitude of said pilot signal so as to measure said
optical power of said first polarized portion.
46. The polarization stabilizing device according to claim 45,
wherein the first monitoring system comprises a splitter for
extracting a power portion of said polarization multiplexed optical
radiation, a polarization splitter for extracting a polarized
portion of said power portion, a photodiode for generating a signal
from said polarized portion of said power portion and a demodulator
for band-pass filtering said signal to obtain said modulation
amplitude of said pilot signal.
47. An optical polarization demultiplexer comprising the
polarization stabilizing device of claim 38, and a polarization
division demultiplexer located downstream the polarization
stabilizing device and oriented parallel or perpendicular to said
defined state of polarization.
48. A polarization division multiplexing system comprising: a
polarization transmitter capable of combining a first and a second
optical channel having orthogonal polarization, wherein the first
channel comprises a pilot signal; a transmission line capable of
transmitting said combined first and second optical channel; and an
optical polarization demultiplexer according to claim 47, optically
coupled to said transmission line and capable of separating said
first and second optical channel.
Description
[0001] The invention relates to polarization stabilization, more
especially to methods and devices for stabilizing with a high
accuracy the polarization state of an optical radiation of
arbitrary, possibly time variant, polarization.
[0002] A polarization stabilizer is a device that transforms an
input optical beam having an input state of polarization (SOP) into
an output optical beam with a predetermined SOP and an optical
power, both not dependent on the input SOP. In general, a defined
SOP is determined by two parameters: the ellipticity and the
polarization azimuth. Such a device is useful, for example, in
coherent optical receivers for matching the SOP between the signal
and the local oscillator, in fiber optic interferometric sensors,
in compensation of polarization mode dispersion of the transmission
line and in optical systems with polarization sensitive components.
An important requirement is the endlessness in control, meaning
that the stabilizer must compensate in a continuous way for the
variations of input SOP.
[0003] In polarization division multiplexing (PolDM) transmission
at least two optical channels, each comprising an optical carrier,
are launched orthogonally polarized in the optical transmission
medium, such as for example an optical transmission fiber. In a
typical solution for PolDM transmission, the two optical carriers
of the at least two orthogonally polarized optical channels are
spectrally closely spaced, such as for example within an optical
spectrum spacing of 50 GHz or within a 25 GHz spacing. In a
preferred configuration, the two carriers, and hence the two
channels, have substantially the same optical wavelength.
Typically, while the reciprocal orthogonality of the state of
polarization is substantially preserved along the propagation into
the transmitting medium, the absolute SOPs of the two channels
randomly fluctuate at a given position along the line, such as for
example at the receiver section.
[0004] In PolDM, a problem arises at the receiving section, or
whenever the two orthogonally polarized channels need to be
polarization demultiplexed. In general, the polarization
demultiplexer is typically a polarization beam splitter, which is
apt to split two orthogonal SOPs. In case of an error in
polarization locking, a misalignment occurs between the SOPs of the
two channels and the orthogonal SOPs divided by the polarization
demultiplexer. In this case a cross-talk is generated due to an
interference between a desired channel and the small portion of the
other non-extinguished channel, which severely degrades the quality
of the received signal. For example, in PolDM systems having the
individual channels intensity modulated with non-return-to-zero
format and directly-detected (IM-DD), the penalty to the
bit-error-rate becomes about 1 dB for cross-talk of about 20 dB.
This means that in case the intensity of the non-extinguished
channel is greater than or equal to about 1% of the intensity of
the demultiplexed channel, the cross-talk becomes a concern.
[0005] Accordingly, in PolDM systems a highly accurate polarization
stabilization of the SOPs of the two polarization multiplexed
channels is needed before polarization demultiplexing. The
cross-talk after polarization demultiplexing is related to the
accuracy of polarization stabilization. In case of a single optical
channel, the accuracy of a polarization stabilizer in terms of
optical power may be expressed through a parameter, called
uniformity error, defined according to
U = I ma x - I m i n I ma x + I m i n , ( 1 ) ##EQU00001##
wherein I.sub.max and I.sub.min are the actual maximum and minimum
optical intensities, in locked operation, of the
polarization-stabilized output radiation of the channel when
varying the input SOP. In general, the smaller is the uniformity
error, the smaller results the cross-talk after demultiplexing. For
example, under simplified conditions, a uniformity error of about
1% gives rise to a cross-talk of about 2%. The patent application
US2004/0016874 discloses (see FIG. 4 thereof) an automatic
polarization controller for a polarization multiplexed optical
pulse train including at least one dither modulation signal, the
polarization controller including a polarization transformer of any
type. A polarization selective element receives the transformed
polarization multiplexed optical pulse train and passes a polarized
optical pulse train including the dither modulation signal. A
detector receives the polarized optical pulse train including the
dither modulation signal and generates a signal that is
proportional to the amplitude of the dither modulation signal. A
feedback control unit generates a control signal that is coupled to
the control input of the polarization transformer.
[0006] The patent application US2002/0191265 discloses (see FIG. 3
thereof) a two-stage electro-optic polarization transformer for
transforming the polarization states of an orthogonally polarized
polarization multiplexed optical signal comprising a first and a
second component. An optical feedback signal is extracted from the
output of the second stage polarization transformer. In one
embodiment, the first and the second components of the polarization
multiplexed optical signal are identified with different dither
frequencies. A mixer generates a signal that has a frequency that
identifies the component of the polarization multiplexed optical
signal.
[0007] The Applicant has noted that the polarization controllers
disclosed in both the documents above directly detect the SOP of
the optical radiation only downstream the polarization transformer
itself (only downstream the second stage in the second document)
and send a single feed-back signal to the feedback control unit.
The methods disclosed thus require complicate elaboration of the
electrical feedback signal and complicate control algorithm,
without adding in precision to the polarization stabilization.
[0008] WO03/014811 patent discloses an endless polarization
stabilizer based on a two-stage configuration wherein the two
stages are controlled independently by an endless polarization
stabilizing method based on a simple feedback control algorithm.
Each stage comprises a pair of birefringent components that each
have fixed eigenaxes and variable phase retardation. The two
birefringent components are variable retarders with finite
birefringence range and respective eigenaxes oriented at
approximately .+-.45 degrees relative to each other. The
endlessness is obtained by commuting the phase retardation of one
retarder, when the retardation of the other retarder reaches a
range limit.
[0009] The Applicant has found that none of the known solutions for
polarization stabilization is at the same time suitable for working
with a polarization multiplexed optical radiation, accurate enough
to meet the specifications in the context of PolDM demultiplexing
and simple enough to be practically feasible and operable. The
Applicant has thus faced the problem of providing a simple,
feasible and highly accurate method and device to stabilize the
state of polarization of a polarization multiplexed optical
radiation having an arbitrary SOP to a predetermined output SOP,
while keeping the output optical power not dependent on the input
SOP. In particular, the Applicant has sought an accuracy suitable
for polarization demultiplexing applications in PolDM systems; for
example the uniformity error is preferably less than or equal to
1%.
[0010] The Applicant has found that in the context of PolDM
systems, in order to achieve highly accurate stabilization of the
SOP, it is advantageous to achieve first an highly accurate
stabilization of one out of the two polarization parameters
(ellipticity and azimuth) and after that a highly accurate full
stabilization of the SOP.
[0011] The Applicant has found that a method and a device based on
two stages each comprising a respective variable birefringent
element, independently controlled by a respective simple and
effective feedback control algorithm, wherein the SOP of the
optical radiation outputting from the first stage is directly
detected through a monitoring system which is sensitive to a pilot
signal contained in the optical radiation, provides polarization
stabilization of a polarization multiplexed optical radiation with
the degree of accuracy needed for polarization demultiplexing in
PolDM systems and the feasibility and operability needed for
industrial application. The Applicant has sought in particular a
method and device for endlessly stabilize the polarization of a
polarization multiplexed optical radiation.
[0012] In some polarization control schemes based on finite range
components, in order to achieve an endless control, it has been
proposed a reset procedure when a component reaches its range limit
so that the output SOP does not change during the reset. Generally,
reset procedures can be problematic in that they are often
associated with complex control algorithms designed to avoid loss
of feedback control during the reset.
[0013] The Applicant believes that a polarization stabilizing
method and device according to the above, wherein each stage
comprises two variable retarders, in combination with a simple and
effective control algorithm, which avoids reset procedure and is
based on the commutation of the first retarder when the second
reaches a retardation range limit, provides the speed, the degree
of accuracy and the feasibility needed for polarization
demultiplexing in PolDM systems.
[0014] The Applicant has found that a polarization stabilizer
device wherein the variable retarders are variable rotators and
each stage also comprises a fixed quarter-wave plate between them,
adds further accuracy and feasibility to polarization stabilization
of a polarization multiplexed radiation.
[0015] In a first aspect, the present invention relates to a method
for stabilizing the state of polarization of a polarization
multiplexed optical radiation comprising an identified channel
which is provided with a pilot signal, the method comprising:
providing to the polarization multiplexed optical radiation a first
controllable polarization transformation to generate a first
transformed optical radiation; measuring a first optical power of a
first polarized portion of said identified channel of the first
transformed optical radiation; controlling, responsively to said
first optical power, the first controllable polarization
transformation so that the identified channel of the first
transformed optical radiation has a predefined polarization
azimuth; providing to the first transformed optical radiation a
second controllable polarization transformation to generate a
second transformed optical radiation; measuring a second optical
power of a second polarized portion of said identified channel of
the second transformed optical radiation; controlling, responsively
to said second optical power, the second controllable polarization
transformation so that the identified channel of the second
transformed optical radiation has a predefined state of
polarization.
[0016] Preferably, said first polarized portion of said identified
channel of the first transformed optical radiation has the
polarization azimuth at .+-.45.degree. with respect to said
predefined polarization azimuth. Advantageously, said second
polarized portion of said identified channel of the second
transformed optical radiation has the state of polarization
parallel or perpendicular to said predefined state of polarization.
Preferably, the first optical power of the first polarized portion
of said identified channel of the first transformed optical
radiation is measured through measuring a modulation amplitude of
said pilot signal. More preferably, said modulation amplitude is
measured after extracting a power fraction from the first
transformed optical radiation, polarizing said power fraction to
generate a polarized power fraction, detecting said polarized power
fraction and pass-band filtering said detected polarized power
fraction to obtain said modulation amplitude.
[0017] Advantageously, also the second optical power of the second
polarized portion of said identified channel of the second
transformed optical radiation is measured through measuring a
modulation amplitude of said pilot signal.
[0018] In order to measure said modulation amplitude, it is
preferable to extract a power fraction from the second transformed
optical radiation, polarize said power fraction to generate a
polarized power fraction, detect said polarized power fraction and
pass-band filter said detected polarized power fraction to obtain
said modulation amplitude.
[0019] The method according to the present invention may further
comprise measuring a third optical power of a third polarized
portion of said identified channel of the first transformed optical
radiation. Preferably, said third polarized portion of said
identified channel of the first transformed optical radiation has
the polarization azimuth orthogonal to the polarization azimuth of
said first polarized portion.
[0020] In a further preferred embodiment, the first controllable
polarization transformation is endlessly varying. Also the second
controllable polarization transformation may be endlessly
varying.
[0021] In a second aspect of the present invention, it is disclosed
a method of demultiplexing a polarization multiplexed optical
radiation, the method comprising any method described above and
further comprising separating the identified channel in the second
transformed optical radiation from a further channel orthogonally
polarized to the identified channel.
[0022] In a third aspect, the present invention is a method of
transmitting a polarization multiplexed optical signal, the method
comprising: providing a pilot signal to an optical channel to
generate an identified channel; polarization multiplexing the
identified channel with a further channel at a first location to
generate a polarization multiplexed optical radiation; propagating
said polarization multiplexed optical radiation at a second
location different from the first location; stabilizing the state
of polarization of the polarization multiplexed optical radiation
at the second location according to any of the method described
above to generate a polarization stabilized optical radiation;
separating the identified channel of the polarization stabilized
optical radiation from the further channel and detecting at least
one of said identified and further channel.
[0023] In a fourth aspect, the invention relates to a device for
stabilizing the state of polarization of a polarization multiplexed
optical radiation comprising an identified channel which is
provided with a pilot signal, the device comprising a first
polarization transformer comprising a first birefringent element
operable to provide a first variable polarization transformation to
the polarization multiplexed optical radiation; a first monitoring
system responsive to said pilot signal and apt to measure the
optical power of a first polarized portion of the identified
channel downstream the first polarization transformer; a controller
apt to control, responsively to the optical power of said first
polarized portion, said first variable polarization transformation
so as to maintain the polarization azimuth of the identified
channel downstream the first polarization transformer at a
predefined azimuth; a second polarization transformer positioned
downstream the first polarization transformer and comprising a
second birefringent element operable to provide a second variable
polarization transformation to the polarization multiplexed optical
radiation; a second monitoring system responsive to said pilot
signal and apt to measure the optical power of a second polarized
portion of the identified channel downstream the second
polarization transformer; and wherein the controller is further apt
to control, responsively to the optical power of said second
polarized portion, said second variable polarization transformation
so as to maintain the state of polarization of the identified
channel downstream the second polarization transformer at a defined
state of polarization.
[0024] Preferably, the first monitoring system is further apt to
measure the optical power of a further polarized portion of the
identified channel downstream the first polarization transformer,
wherein said further polarized portion is orthogonal to the first
polarized portion.
[0025] The first polarization transformer may further comprise a
third birefringent element operable to provide a third variable
polarization transformation to the polarization multiplexed optical
radiation. In this case, it is preferable that the controller is
configured to switch the third variable polarization transformation
between first and second values when the first variable
polarization transformation reaches a predefined threshold value,
in order to provide a reset-free endless control. Preferably, each
of the first birefringent element and the third birefringent
element comprises a respective variable rotator and the first
polarization transformer further comprises a quarter-wave plate
optically interposed between the first and the third birefringent
element.
[0026] The second polarization transformer may further comprise a
fourth birefringent element operable to provide a fourth variable
polarization transformation to the polarization multiplexed optical
radiation. In this case, the controller is configured to switch the
fourth variable polarization transformation between third and
fourth values when the second variable polarization transformation
reaches a predefined threshold value in order to provide a
reset-free endless control.
[0027] Advantageously, the first monitoring system is configured to
measure a modulation amplitude of said pilot signal so as to
measure said optical power of said first polarized portion.
Preferably, the first monitoring system comprises a splitter for
extracting a power portion of said polarization multiplexed optical
radiation, a polarization splitter for extracting a polarized
portion of said power portion, a photodiode for generating a signal
from said polarized portion of said power portion and a demodulator
for band-pass filtering said signal to obtain said modulation
amplitude of said pilot signal.
[0028] In a fifth aspect, the invention relates to an optical
polarization demultiplexer comprising the polarization stabilizing
device described above and a polarization division demultiplexer,
such as e.g. a polarization beam splitter, located downstream the
polarization stabilizing device and oriented parallel or
perpendicular to said defined state of polarization.
[0029] In a sixth aspect, the invention relates to a polarization
division multiplexing system comprising a polarization transmitter
apt to combine a first and a second optical channel having
orthogonal polarization, wherein the first channel comprises a
pilot signal; a transmission line apt to transmit said combined
first and second optical channel; and an optical polarization
demultiplexer describe above, optically coupled to said
transmission line, and apt to separate said first and second
optical channel.
[0030] For a better understanding of the invention and to show how
the same may be carried into effect reference is now made by way of
example to the accompanying drawings.
[0031] FIG. 1. Schematic drawing of a polarization division
multiplexing optical system according to one aspect of the present
invention.
[0032] FIG. 2. Schematic drawing of a base architecture of the
polarization stabilizer device according to the present
invention.
[0033] FIG. 2a. Schematic drawing of an alternative configuration
of the first stage of the polarization stabilizer device of FIG.
2.
[0034] FIG. 3. Schematic drawing of a first exemplary embodiment of
the polarization stabilizer device of FIG. 2.
[0035] FIG. 4. Poincare sphere representation of a polarization
stabilizer device according to the first embodiment of the present
invention.
[0036] FIG. 5a. Four exemplary points on the Poincare sphere
representing four exemplary input SOPs to the polarization
stabilizer of the present invention.
[0037] FIGS. 5b-5e. SOP transformations on the Poincare sphere
generated by the first embodiment of the present invention
polarization stabilizer corresponding to the four input SOPs of
FIG. 5a
[0038] FIG. 6a-6c. SOP transformations on the Poincare sphere
generated by the first embodiment of the present invention
polarization stabilizer.
[0039] FIG. 7. Diagram of bit error rate (BER) versus power at the
receiver of an optical system employing the present invention
polarization stabilizer device with different pilot tone modulation
index.
[0040] FIG. 8. Diagram of bit error rate (BER) versus power at the
receiver of an optical system with and without polarization
stabilization according to the present invention.
[0041] FIG. 9. Schematic drawing of a second exemplary embodiment
of the polarization stabilizer device of FIG. 2.
[0042] FIG. 10. Schematic drawing of a third exemplary embodiment
of the polarization stabilizer device of FIG. 2.
[0043] FIG. 1 schematically shows a polarization division
multiplexing system 1 in accordance with one aspect of the present
invention.
[0044] A transmitter section TX is apt to encode data information
into a polarization multiplexed optical radiation comprising two
optical channels orthogonally polarized. The transmitter section TX
may include optical sources (e.g. lasers), modulators (e.g.
electro-optic modulators), wavelength multiplexers, polarization
multiplexers, optical boosters, etc. One of the two channels,
hereinafter referred to as the identified channel, is provided with
a pilot signal which may serve to uniquely identify said channel.
Optionally, the other of the two channels may also be provided with
a second pilot signal uniquely identifying it.
[0045] The pilot signal may be a superimposed modulation such as
for example an amplitude or intensity modulation, a phase
modulation, an optical frequency modulation or a polarization
modulation, or it may be an identifying clock, for example an
identifying bit-clock. The superimposed modulation may follow any
given waveform, such as for example an harmonic wave (hereinafter
called pilot tone in case of intensity modulation) or a square wave
(usually called dither). The frequency of modulation of the
superimposed modulation should be low enough with respect to the
data modulation rate (bit-rate) in order not to degrade the
transmission quality. For example, in case of a bit-rate of 622
Mb/s or greater, it is advantageous to set the pilot signal
frequency less than or equal to 10 MHz. On the other end, the
frequency of modulation of the pilot signal should be high enough
to differ from the continuous (zero frequency) spectral component.
A possible range for the pilot signal frequency is from 1 kHz to 10
MHz, including the ends of range.
[0046] The two channels are launched into an optical transmission
line 2 with mutually orthogonal state of polarization. The optical
transmission line 2 may include for example an optical cable
comprising optical fibers. Optical line amplifiers LA, such as for
example EDFAs, may be distributed along the optical transmission
line 2. Also, one or more optical processing units OPU may be
placed along the line 2 in order to perform operations on the
optical signal such as routing, regeneration, add and/or drop,
switching and the like. A receiver section RX is placed at the end
of the transmission line 2 or whenever the optical signal needs to
be received (e.g. at the OPU), in order to convert the optical
signal into an electrical signal. It may comprises optical
pre-amplifiers, optical filters, photodetectors, electrical
filters, etc.
[0047] A polarization stabilizer device 100 according to the
present invention is placed upstream the receiver section RX in
order to stabilize the SOP of the polarization multiplexed optical
radiation to a defined SOP before inputting the receiving section
RX. In other words, the SOP of one of the two optical channels
inputting the polarization stabilizer device 100 is converted to a
defined SOP and consequently the SOP of the other of the two
optical channels is uniquely stabilized to a SOP orthogonal to the
defined SOP. Throughout the present description, reference will be
made to the SOP of the identified channel, being the SOP of the
other optical channel uniquely determined.
[0048] In case a wavelength division multiplexing (WDM) technique
is used in combination with PolDM in the optical transmission
system 1, each WDM carrier wavelength comprises two orthogonally
polarized channels wherein one channel of each couple is identified
by a pilot signal. In this case, a wavelength demultiplexer D-MUX
may be placed upstream the polarization stabilizer 100 in order to
separate, at least partially, the different optical wavelengths.
Advantageously, a polarization selective element PS, for example a
polarization division demultiplexer such as a polarization beam
splitter having its azimuth oriented parallel or perpendicular to
the defined SOP, may be placed at the output end of the
polarization stabilizer device 100 in order to separate the two
polarization multiplexed channels. The polarization selective
element PS may be integrated either within the polarization
stabilizer device 100 or within the receiver section RX.
[0049] In case the two orthogonally polarized optical channels are
closely spaced in the optical spectrum without overlapping
(polarization-interleaved WDM), it is preferable to superimpose a
pilot signal to each WDM channel. For example, odd channels have a
first pilot signal and even channels have a second pilot signal
(e.g. having frequency different from the first one). In this case,
the wavelength demultiplexer D-MUX placed upstream the polarization
stabilizer device 100 passes the desired WDM channel and one or
more undesired adjacent optical channels. The desired WDM channel
has a SOP orthogonal to the SOP of the adjacent channels. In
polarization-interleaved WDM the polarization selective element PS
is advantageously a linear polarizer. The polarization stabilizer
device 100 thus acts to align the SOP of the desired WDM channel to
the polarizer by making use of the pilot signal of the desired
channel. The residual portion of the adjacent WDM channels are thus
filtered out by the polarizer.
[0050] FIG. 2 is a schematic representation of a base architecture
of the polarization stabilizer device 100 according to the present
invention.
[0051] The device 100 comprises a first and a second stage 200 and
300.
[0052] The device 100 has a principal beam path `x` along which a
polarization multiplexed optical radiation is received as an input
optical radiation of arbitrary state of polarization of the
identified channel (labeled SOP.sub.IN in the figure); the
radiation then traverses the first stage 200 and outputs the first
stage 200 with a SOP (labeled SOP.sub.INT) having the polarization
azimuth at a predefined value. Conventionally, the polarization
azimuth will range from -90.degree. to +90.degree. modulus
180.degree.. For the purpose of the present invention, a predefined
value of the polarization azimuth means a couple of angular values
differing of 90.degree.. Examples of predefined polarization
azimuth are (-30.degree.,+60.degree.) or (0.degree.,+90.degree.) or
(-45.degree.,+45.degree.). It is noted that also the polarization
azimuth of the other orthogonally polarized channel is at the same
predefined value. The polarization multiplexed optical radiation
then traverses the second stage 300 and is emitted from the device
100 as an optical radiation having a stabilized defined SOP of the
identified channel (labeled SOP.sub.OUT) and an optical power not
depending on the input SOP. Without loss of generality, the defined
SOP may be the linear vertical SOP having defined vertical azimuth
and defined zero ellipticity.
[0053] The first stage 200 comprises a polarization transformer PT1
which is apt to give to the optical radiation propagating through
it a first controllable polarization transformation. The
polarization transformer PT1 may comprise a birefringent element
BE1 or a combination of single birefringent elements including
BE1.
[0054] The first stage 200 also comprises a monitoring system MS1
which is responsive to the pilot signal of the identified channel
and is apt to measure uniquely the optical power of a polarized
portion of the identified channel outputting from the polarization
transformer PT1. Throughout the present description, the term
"polarized portion" or "polarized component" means the projected
component of the optical radiation along a given SOP. For sake of
clarity, in case of deviation of the optical radiation, for example
a reflection by a beam splitter, the reference system for the SOP
is accordingly transported.
[0055] The first stage 200 also comprises a controller CTRL1 (e.g.
an electronic controller or a computer) which is apt to control the
first controllable polarization transformation, given by the
polarization transformer PT1 through e.g. the birefringent element
BE1, in response to the optical power of the polarized portion of
the identified channel measured by the monitoring system MS1, so
that the azimuth of the SOP of the identified channel outputting
from the polarization transformer PT1 remains at a target
predefined value. The controller CTRL1 is connected to the monitor
system MS1 and to the polarization transformer PT1. The second
stage 300 comprises a polarization transformer PT2 which is apt to
give to the optical radiation propagating through it a second
controllable polarization transformation. The polarization
transformer PT2 may comprise a birefringent element BE2 or a
combination of birefringent elements including BE2. The second
stage 300 also comprises a monitoring system MS2 which is
responsive to the pilot signal of the identified channel and is apt
to measure uniquely the optical power of a polarized portion of the
identified channel outputting from the polarization transformer
PT2.
[0056] The second stage 300 also comprises a controller CTRL2 (e.g.
an electronic controller or a computer) which is apt to control the
second controllable polarization transformation, given by the
polarization transformer PT2 through e.g. the birefringent element
BE2, in response to the optical power of the polarized portion of
the identified channel measured by the monitoring system MS2, so
that the SOP of the identified channel outputting from the
polarization transformer PT2 remains at a defined SOP. The
controller CTRL2 is connected to the monitor system MS2 and to the
polarization transformer PT2.
[0057] Although, for the sake of clarity, two separate controllers
CTRL1 and CTRL2 have been described and represented in FIG. 2, it
can be appreciated that a single controller can advantageously be
employed, connected in input to the monitoring systems MS1 and MS2
and in output to the polarization transformers PT1, PT2.
Advantageously, a polarization division demultiplexer PDD, which is
a particular embodiment of the polarization selective element PS of
FIG. 1, may be placed along the main beam path `x` downstream the
second stage 300 to separate the polarization multiplexed optical
channels. For example, a polarizing beam splitter oriented with its
azimuth extending parallel or perpendicular to the defined output
azimuth.
[0058] The polarization transformers PT1 and PT2 are placed on the
principal beam path `x`. The birefringent element BE1 and BE2 may
be any kind of birefringent element apt to give a variable
polarization transformation, such as for example a variable
retarder having fixed eigenstates and variable phase retardation,
or birefringent element having fixed phase retardation and variable
eigenstates (e.g. rotating axes), or variable eigenstates and
variable phase retardation.
[0059] In general any physical mechanism producing a birefringence
can be exploited to realize the birefringent elements used in the
polarization stabilizer device 100 of the present invention. For
example, they may be based on the magneto-optic effect (e.g.
Faraday rotator), the electro-optic effect (such as the nematic
liquid-crystal retarders or the electro-optic crystals belonging to
the symmetry point group of the zincblende such as zinc sulfide
(ZnS) with its ternary or higher order compounds (e.g. ZnSSe);
cadmium telluride (CdTe) with its ternary or higher order compounds
(e.g. CdZnTe); gallium arsenide (GaAs) with its ternary or higher
order compounds (e.g. AlGaAs, InGaAsP); and the like) or the
elasto-optic effect (such as squeezers).
[0060] The monitor systems MS1 and MS2 are associated to the
principal beam path They are designed to be responsive to the pilot
signal. Accordingly, they are apt to identify the identified
channel through the pilot signal and to measure only the optical
power of the identified channel.
[0061] In FIG. 2, it is shown an exemplary embodiment of the
monitoring systems MS1 and MS2 apt to be used in connection with a
superimposed amplitude or intensity modulation as the pilot signal
of the identified channel.
[0062] Accordingly, the monitoring systems MS1 and MS2 of the first
and second stage 200 and 300 may comprise a polarization
insensitive beam-splitter, respectively BS1 and BS2, arranged in
the beam path `x` downstream the respective polarization
transformer PT1 and PT2. BS1 and BS2 are apt to extract a small
fraction of the optical radiation outputting from the respective
polarization transformer PT1 and PT2. For minimum losses, the
extracted portion of the radiation should be vanishingly small.
However, in practice, the diverted portion needs to be large enough
to provide a reasonable signal-to-noise ratio for subsequent
processing associated with the control loop. A diverted power
fraction of between 1 and 10% may be typical. It will be
appreciated that other optical components can provide the same
function of extracting a small fraction of the beam, for example an
optical fiber coupler.
[0063] The monitor system MS1 of the first stage 200 may comprise a
polarizing beam splitter PBS optically connected to the beam
splitter BS1, as shown in FIG. 2. The PBS is apt to receive the
optical radiation extracted by the beam splitter BS1. The azimuth
of the PBS is approximately at .+-.45.degree. with respect to the
predefined azimuth. For example, at a predefined azimuth of
(-30.degree.,+60.degree.) corresponds a PBS azimuth of +15.degree.
or -75.degree.. In other words, the PBS is apt to separate a
linearly polarized portion of the extracted optical beam having an
azimuth at +45.degree. to the predefined azimuth from a linearly
polarized portion of the optical radiation having an azimuth at
-45.degree. to the defined azimuth.
[0064] Throughout the present description, a polarization beam
splitter PBS is functionally equivalent, and interchangeable, to a
polarization insensitive beam splitter followed by two orthogonally
oriented linear polarizers, one for each output of the polarization
insensitive beam splitter. Optical fiber or optical waveguides
components can also be used to provide the same function.
[0065] It is noted that at each output of the PBS, the polarized
portions of both the polarization multiplexed optical channels are
present and overlapping.
[0066] A first and a second photodiode PD1 and PD2 may be optically
connected to the polarizing beam splitter PBS, one for each output
of the PBS. They are apt to detect the two polarized components of
the optical radiation outputting respectively from the two outputs
of the PBS and to generate respective signals responsive of the
optical power of these two polarized components.
[0067] In particular applications, for example when the power of
the input optical beam is known and can be held constant, either
photodiode PD1 or photodiode PD2 may be omitted. In this case, the
polarizing beam splitter PBS may be replaced by a fixed linear
polarizer oriented either at +45.degree. or -45.degree. to the
predefined azimuth.
[0068] The monitor system MS2 of the second stage 300 may comprise
a linear polarizer P2, preferably fixed, optically connected at the
reflected output of the beam splitter BS2, as shown in FIG. 2. The
polarizer P2 is apt to receive the optical radiation extracted by
the beam splitter BS2. The azimuth of the P2 is approximately
parallel or perpendicular with respect to the defined output SOP.
In other words, the polarizer P2 is apt to pass a linearly
polarized portion of the extracted optical beam having a SOP
parallel or perpendicular to the defined SOP.
[0069] A photodiode PD3 may be connected to the output end of the
polarizer P2 and is apt to measure the optical power of the
extracted polarized portion and generate a signal responsive of
this power.
[0070] A first, a second and a third demodulator DM1, DM2 and DM3
may be connected to the first, second and third photodiode PD1, PD2
and PD3, respectively. The first, second and third demodulator DM1,
DM2 and DM3 are apt to receive respective signal from first, second
and third photodiode PD1, PD2 and PD3 and to respond to the pilot
signal. For example, in case a pilot tone (sinusoidal amplitude
modulation) is used for the identified channel, each demodulator
executes a pass-band filtering of the electrical signal generated
by the corresponding photodiode, around the pilot tone frequency.
Such a filtered signal, neglecting the noise terms, can be
expressed as a sinusoid at a pilot tone frequency f.sub.PT with an
amplitude of modulation A.sub.i (t) according to:
s.sub.i(t)=A.sub.i(t)sin(2.pi.f.sub.PTt) (1)
where i=1, 2, 3 refer respectively to the first, second and third
demodulator DM1, DM2 and DM3. The i-th pilot tone amplitude
A.sub.i(t) is directly proportional to the optical intensity of
solely the polarized portion of the identified channel incident on
the corresponding i-th photodiode. The action of the demodulators
is to measure these pilot tone amplitudes A.sub.i(t), carrying the
information about the SOP of the identified channel and used by the
controllers CTRL1 and CTRL2 for the SOP stabilization. Such a
demodulator DM1, DM2 or DM3 can be realized by using any electrical
scheme among those well known in radio engineering for detecting an
amplitude modulation of a carrier. For example the demodulator may
be based on envelope detection or coherent detection schemes.
[0071] The first, second and third demodulator DM1, DM2 and DM3 may
generate respective output signals V.sub.1, V.sub.2 and V.sub.3,
indicative of the respective pilot tone amplitudes A.sub.1, A.sub.2
and A.sub.3, which in turn are indicative of the optical powers of
the respective polarized portions of the identified channel. It
will be appreciated that these signals may be in electronic form,
with the photodiodes being optoelectronic converters and the
demodulators being electronic circuits. However, it will also be
appreciated that these processing elements could be embodied with
all-optical components of the same functionality. This may be
desirable for stabilizing extremely high frequency polarization
instabilities where all-optical power sensing and control
processing could be performed. In addition, the signals V.sub.1,
V.sub.2 and V.sub.3 may also be radio signals. It will be also
appreciated that demodulation of the pilot signal may be performed
directly by the photodiodes PD1, PD2 and PD3.
[0072] In those applications, described above, wherein either
photodiode PD1 or photodiode PD2 may be omitted, also the
respective demodulator DM1 or DM2 and the respective signal V.sub.1
or V.sub.2 may be omitted.
[0073] The function of the first stage 200 of the device 100 is to
transform any input SOP of the identified channel into an
elliptical output SOP (SOP.sub.INT) with major axis (said
polarization azimuth) at a predefined azimuth.
[0074] In operation, the input polarization multiplexed optical
radiation having an identified channel traverses the polarization
transformer PT1. The polarization transformer gives to the optical
radiation a variable controllable polarization transformation, such
that the SOP of the identified channel is transformed from
SOP.sub.IN to SOP.sub.INT, outputting from the polarization
transformer PT1, wherein SOP.sub.INT has an azimuth at a predefined
value.
[0075] A feedback control loop is designed to lock the polarization
azimuth of the SOP (SOP.sub.INT) of the identified channel
outputting from the polarization transformer PT1 to the target
azimuth value (as defined above, a couple of values mutually
orthogonal). Accordingly, the monitoring system MS1 measure the
optical power of a polarized portion of solely the identified
channel, wherein the polarized portion is preferably the linearly
polarized portion having an azimuth at .+-.45.degree. to the
defined azimuth. The monitoring system MS1 may generate an output
signal V.sub.1 indicative of such optical power.
[0076] The controller CTRL1 of the first stage 200 is connected to
the monitoring system MS1 and it is apt to receive the signal
V.sub.1. The controller CTRL1 has an output connected to the
birefringent element BE1 of the polarization transformer PT1. The
controller CTRL1 is apt to generate an output control signal
(labeled .phi..sub.1 in FIG. 2), responsive to, the signal V.sub.1,
according to a control algorithm. The output control signal
.phi..sub.1 is suitable to be sent to, and to control the
polarization transformation by, the birefringent element BE1 in
order to lock the polarization azimuth of the identified channel
outputting from the polarization transformer PT1 at the defined
azimuth.
[0077] The control algorithm is a simple cyclic control algorithm
that can be implemented on a digital PC-based controller (CTRL1),
or any other suitable hardware, firmware, software or combination
thereof. An all-optical processor could also be used for the
controller.
[0078] Preferably, the control algorithm contains a calculation of
an error value, related to the signal V.sub.1, which is related to
the displacement of the polarization azimuth of the identified
channel outputting from the polarization transformer PT1 from the
defined azimuth value. The aim of the control algorithm and, more
in general, of the control feedback loop is to minimize the above
error.
[0079] For example, the error may be defined so that it is ideally
zero when the linearly polarized components of the identified
channel (between the two stages 200 and 300) at +45.degree. and at
-45.degree. to the defined azimuth have equal optical power.
[0080] The minimization of the error is achieved by controlling the
polarization transformation applied by the birefringent element
BE1. The polarization transformation applied by the birefringent
element BE1 is typically varied in a continuous or quasi-continuous
manner, with a discretization that follows from the stepwise
incremental nature of the computer-implemented control scheme. It
is convenient that the steps in the polarization transformation
have a constant absolute value, although non-constant steps, for
example dependent on the absolute value of the polarization
transformation, could be used. In general, the smaller the step,
the better the stabilization (smaller uniformity error), but a
trade-off with the stabilization speed need to be considered.
[0081] At each control period or step the signal control
.phi..sub.1 of BE1 may be changed so that the respective
polarization transformation changes by a constant quantity. At each
step the control algorithm calculates the error: if the error at
the current step becomes larger than the error at the previous
step, then the sign of the polarization transformation variation is
changed, else the sign is not changed.
[0082] The elliptical SOP with fixed axes (SOP.sub.INT), obtained
as output of the first stage 200, is transformed by the second
stage 300 into a fixed linear SOP with optical power independent
from the input SOP. In detail, the polarization multiplexed optical
radiation outputting the first stage 200 traverses the polarization
transformer PT2. The polarization transformer PT2 gives to the
optical radiation a further controllable polarization
transformation, such that the SOP of the identified channel is
transformed from SOP.sub.INT to SOP.sub.OUT, outputting from the
polarization transformer PT2, wherein SOP.sub.OUT is a defined SOP
(defined azimuth and defined ellipticity). For sake of clarity, the
defined azimuth of the defined output SOP may be different from the
predefined azimuth described above.
[0083] The feedback control loop of the second stage 300 of FIG. 2
is designed to lock the SOP (SOP.sub.OUT) of the identified channel
outputting from the polarization transformer PT2 to the target SOP
in a way similar to the feed-back control loop of the first stage
200. To this purpose, the monitoring system MS2 measure the optical
power of a polarized portion of solely the identified channel,
wherein the polarized portion is preferably the linearly polarized
portion parallel or perpendicular to the defined SOP. The monitor
system MS2 generates respective output signal V.sub.3.
[0084] The controller CTRL2 of the second stage 300 is connected to
the monitor system MS2 and it is apt to receive the signal V.sub.3.
The controller CTRL2 has an output connected to the birefringent
element BE2 of the polarization transformer PT2. The controller
CTRL2 is apt to generate an output control signal (labeled
.phi..sub.2 in FIG. 2), responsive to the signal V.sub.3, according
to a control algorithm similar to that described with reference to
the first stage 200. The output control signal .phi..sub.2 is
suitable to be sent to, and to control the polarization
transformation by, the birefringent element BE2 in order to lock
the state of polarization of the identified channel outputting from
the polarization transformer PT2 at a defined SOP.
[0085] The controller CTRL2 may execute the same control algorithm
as the first stage 200, the only difference being that the error is
correlated to V.sub.3. The aim of the feed-back is to minimize or
maximize (depending on the azimuth orientation of the fixed
polarizer P2) this error.
[0086] The fact that the first stage 200 is controlled
independently of the second stage 300 is highly advantageous, since
the provision of two stages does not lead to any additional
complexity to the control, since no time synchronization between
the first and second respective controllers CTRL1 and CTRL2 is
required.
[0087] Separate controllers CTRL1 and CTRL2 are shown for the first
200 and second stage 300 of FIG. 2, consistent with the functional
independence of the control algorithms of the two stages from one
another. However, it will be understood that the two controllers
could be embodied in a single hardware, firmware or software
unit.
[0088] Therefore, the device 100 may comprise a single controller
apt to control, responsively to the optical power of the polarized
portion of the identified channel downstream the polarization
transformer PT1, the variable polarization transformation provided
by the polarization transformer PT1 so as to maintain the
polarization azimuth of the identified channel downstream the first
polarization transformer PT1 at the predefined azimuth, and is
further apt to control, responsively to the optical power of the
polarized portion of the identified channel outputting from the
polarization transformer PT2, the variable polarization
transformation provided by the polarization transformer PT2 so as
to maintain the state of polarization of the identified channel
downstream the second polarization transformer PT2 at a predefined
state of polarization.
[0089] FIG. 2a shows a possible alternative configuration of the
first stage 200 of the polarization stabilizer device 100 which is
suitable to be used in combination with a superimposed intensity
modulation as pilot signal of the identified optical channel. The
alternative configuration of the first stage 200 shown in FIG. 2a
essentially differs from the configuration of the first stage 200
shown in FIG. 2 in the monitoring system MS1. The devices of stage
200 of FIG. 2a that are identical to the devices of stage 200 of
FIG. 2 will be indicated with the same reference numeral.
[0090] A polarization insensitive beam-splitter BS' (e.g. with a
90/10 split ratio) may be arranged in the beam path `x` and it is
apt to extract a small fraction (e.g. 10% in this example, or 1%)
of the input optical beam. The extracted fraction of the input
optical beam is directed to a photodiode PD' which is apt to
measure the power of the extracted fraction. A demodulator DM' is
connected to the output of photodiode PD'.
[0091] The beam splitter BS' shown in FIG. 2a is located upstream
the polarization transformer PT1, but possible variations would be
to arrange the polarization insensitive beam splitter BS' along the
beam path `x` either between the polarization transformer PT1 and
the beam splitter BS1 or downstream the beam splitter BS1.
Alternatively, the beam splitter BS' can be also located between
the beam splitter BS1 and the polarizer P1.
[0092] As shown in FIG. 2a, a fixed linear polarizer P1 is apt to
receive the optical radiation extracted by the beam splitter BS1.
The azimuth of the linear polarizer P1 may be oriented either at
+45.degree. or -45.degree. to the predefined azimuth (couple of
angular values). A photodiode PD1, with its associated demodulator
DM1, is optically connected to P1 so that it is apt to measure the
power of the polarized component transmitted by P1.
[0093] The principle of operation of the first stage 200 of FIG. 2a
is similar to the one exemplarily described for the first stage 200
of FIG. 2. It is provided a monitoring system MS1 comprising
elements (e.g. BS1, P1, PD1, DM1) having the function of extracting
a polarized portion (e.g. at +45.degree. or -45.degree. to the
defined azimuth) of the optical radiation outputting from the
polarization transformer PT1 and generating a signal V.sub.1
responsive to the optical power of the extracted polarized portion
of solely the identified channel, via a demodulation operation
performed, e.g., by a demodulator DM1. The detecting system of the
first stage 200 of FIG. 2a further comprises elements (e.g. BS',
PD', DM') having the function of extracting a portion of the
optical radiation along the beam path `x` and generating a signal
V' responsive to the pilot signal and indicative of the optical
power of solely the identified channel propagating along the beam
path `x`.
[0094] A controller CTRL1 generates an error value by comparing the
optical power of the extracted polarized portion of the identified
channel (represented by V.sub.1) with a value which is the expected
value for this polarized component when the identified channel
outputting from the polarization transformer PT2 has a polarization
azimuth at a defined value. Such expected value is calculated based
on the signal V'. For example, the error value may be defined as
.epsilon.=|V'-.alpha.V.sub.1|, wherein a serves for the comparison
of the extracted polarized portion (V.sub.1) with an expected value
derived from V'. This error serves, through a cyclic feedback
algorithm similar to the one described above, to control the proper
polarization transformation at each control step.
[0095] Throughout the following description, it will be exemplarily
assumed that the identified channel is identified by a pilot tone,
that is to say a superimposed sinusoidal amplitude modulation,
preferably having low amplitude and low frequency.
[0096] A first embodiment of the polarization stabilizer device of
FIG. 2 will now be described with reference to FIG. 3. The same
reference numerals will be used for elements in FIG. 3 identical to
corresponding elements in FIG. 2. This embodiment is endless and
has no intrinsic loss. In other words, in perfect lossless
operation of the components of the optical device 100, the
polarization stabilized output optical radiation can potentially
have up to the full power of the input optical radiation.
[0097] The device 100 of FIG. 3 is apt to receive a polarization
multiplexed optical radiation as an input optical radiation having
an identified channel comprising a pilot signal with arbitrary
state of polarization (labeled SOP.sub.IN in the figure). The
polarization multiplexed optical radiation is emitted from the
device 100 as an optical radiation having a stabilized defined SOP
of the identified channel (labeled SOP.sub.OUT) and an optical
power not depending on the input SOP. The defined SOP has a defined
azimuth and a defined ellipticity. Without loss of generality, the
defined SOP may be the linear vertical SOP having the defined
azimuth vertical and the defined ellipticity zero.
[0098] The device 100 comprises a first and a second stage 200 and
300.
[0099] The polarization multiplexed optical radiation traverses the
first stage 200 and outputs the first stage 200 with a SOP (labeled
SOP.sub.INT) having the polarization azimuth at .+-.45.degree. with
respect to the defined output azimuth (i.e.
(-45.degree.,+45.degree.) having assumed a vertical output
azimuth). The optical radiation then traverses the second stage
300.
[0100] The first stage 200 comprises a polarization transformer PT1
comprising a first and second variable rotators VPR1 and VPR2,
which are variable circularly birefringent elements with
controllable phase retardations .PHI..sub.1 and .PHI..sub.2,
respectively. A (polarization) rotator can be seen as a
birefringent element with circular eigenstates, that is an element
that rotates the azimuth of the SOP. A circularly birefringent
element giving a phase retardation .PHI. between the circular
eigenstates causes a rotation of an angle .PHI./2 of the
polarization azimuth. The first variable rotator VPR1 has an
associated controllable phase retardations .PHI..sub.1 which may
have a finite range, i.e. it may have an upper limit or a lower
limit, or both. Advantageously, it may assume one out of two
retardation values which are integer multiples of .pi. radians and
differ by an odd integer multiple of .pi. radians. The second
variable rotator VPR2 has an associated controllable phase
retardations .PHI..sub.2 which may have a finite range.
Advantageously, it may smoothly vary at least in a range from k.pi.
to (k+k').pi. radians, wherein k is an integer and k' is an odd
integer.
[0101] In a preferred configuration, the variable rotators VPR1 and
VPR2 are variable Faraday rotators, i.e. variable polarization
rotators which make use of the magneto-optical Faraday effect and
wherein the magnetic field applied to a magneto-optical material is
varied.
[0102] The polarization transformer PT1 also comprises a
quarter-wave plate WP1 optically interposed between the first and
second variable rotators VPR1 and VPR2 and having the eigenaxes
oriented at .+-.45 degree with respect to the defined azimuth. The
quarter-wave plate WP1, as well any other component in the present
invention, may be replaced by a technical equivalent, such as a
combination of birefringent elements performing the same function,
without exiting from the scope of the present invention. In a
preferred configuration, the polarization transformer PT1 consists,
for what concerns the optical birefringent elements, only of the
first and second variable rotators VPR1 and VPR2 and the
quarter-wave plate WP1 optically interposed therebetween. Such a
polarization transformer PT1 is advantageous due to its simplicity
and consequently low insertion loss, high feasibility and high
accuracy.
[0103] A monitoring system MS1 is provided to the first stage 200
in all similar to that described with reference to FIG. 2.
Alternatively, the monitoring system MS1 of FIG. 2a may be used. It
comprises a polarization insensitive beam-splitter BS1 to extract a
small fraction of the optical radiation outputting from the second
variable rotator VPR2, a polarizing beam splitter PBS having
azimuth approximately parallel or perpendicular to the defined
azimuth, a first and a second photodiode PD1 and PD2, and a first
and a second demodulator DM1 and DM2 apt to generate respective
signals V.sub.1 and V.sub.2 responsive of the optical power of,
respectively, the two polarized components of solely the identified
channel outputting form the PBS.
[0104] A controller (e.g. electronic) CTRL1 is connected to the
first and second demodulator DM1 and DM2 and it is apt to receive
the signals V.sub.1 and V.sub.2. The controller CTRL1 has first and
second outputs connected respectively to the first and second
rotators VPR1 and VPR2. The controller CTRL1 is apt to generate
output control signals (labeled .phi..sub.1 and .phi..sub.2 in FIG.
3), responsive to the signals V.sub.1 and V.sub.2, according to a
control algorithm described further below. The output control
signals .phi..sub.1 and .phi..sub.2 are suitable to be sent to, and
to control the phase retardations .PHI..sub.1 and .PHI..sub.2 of,
the rotators VPR1 and VPR2, respectively.
[0105] In particular applications, photodiode PD2 (and consequently
demodulator DM2) may be omitted. In this case, the polarizing beam
splitter PBS may be replaced by a fixed linear polarizer oriented
either parallel or perpendicular to the defined azimuth.
[0106] The function of the first stage 200 is to transform any
input SOP of the identified channel into an elliptical output SOP
(SOP.sub.INT) with principal axes at .+-.45 degrees to said defined
azimuth.
[0107] In operation, the input polarization multiplexed optical
radiation traverses sequentially the first variable rotator VPR1,
the quarter-wave plate WP1 and the second variable rotator VPR2.
The first variable rotator VPR1 and the second variable rotator
VPR2 rotate the azimuth of the optical radiation by respectively a
first and a second variable angle .PHI..sub.1/2 and .PHI..sub.2/2,
such that, in combination with the fixed action of the quarter-wave
plate WP1, the SOP of the identified channel outputting from the
second variable rotator VPR2 (SOP.sub.INT) has an azimuth at .+-.45
degrees with respect to the defined output azimuth.
[0108] A feedback control loop is designed to lock the polarization
azimuth of the SOP (SOP.sub.INT) of the identified channel
outputting from the second rotator VPR2 to the target azimuth value
equal to .+-.45 degrees with respect to the defined azimuth. The
polarization insensitive beam splitter BS1 diverts a portion of the
beam out of the main beam path `x`. The diverted portion of the
beam is then received by the polarizing beam splitter PBS which
splits the diverted beam portion into its two orthogonal
polarization components, which are supplied to, and detected by,
the respective photodiodes PD1 and PD2. The demodulators DM1 and
DM2 act on the signals generated by the photodiodes PD1 and PD2 and
they supply respective signals V.sub.1 and V.sub.2 as input signals
to the controller CTRL1.
[0109] The controller CTRL1 executes an algorithm described below
and generates the two signals .phi..sub.1 and .phi..sub.2,
responsive of signals V.sub.1 and V.sub.2, controlling the phase
retardations .PHI..sub.1 and .PHI..sub.2 respectively of VPR1 and
VPR2. The algorithm may contain a calculation of an error value
which is related to the displacement of the polarization azimuth of
the optical radiation outputting from the second variable rotator
VPR2 from the target azimuth value. The aim of the control loop is
to minimize the above error.
[0110] For example, the error may be defined as
.epsilon.=|V.sub.1-.alpha.V.sub.2|, where the parameter .alpha. is
determined so that the error is ideally zero when the linearly
polarized components of the identified channel parallel and
perpendicular to the defined azimuth have equal optical power. This
condition is equivalent to the target of an elliptical SOP.sub.INT
with principal axes at .+-.45 degrees to said defined azimuth. For
example, considering the case of the stabilizer device 100 of FIG.
3 having an ideal PBS and photodiodes PD1 and PD2 having equal
responsivities, the value of .alpha. may be chosen equal to 1. In
general, different devices may have different values for the
parameter .alpha..
[0111] In those applications, described above, wherein photodiode
PD2 may be omitted, there is acquired at each control period of the
feedback loop only one signal V.sub.out responsive of the optical
power of a polarized component of solely the identified channel and
the error is defined as .epsilon.=|V.sub.out-V.sub.ref|, where
V.sub.ref is set via the CTRL1 taking into account the input
optical power and the behavior of the optical elements, e.g. their
insertion losses.
[0112] The minimization of the error is achieved by controlling the
phase retardations .PHI..sub.1 and .PHI..sub.2 of the two variable
rotators VPR1 and VPR2. The phase retardation .PHI..sub.2 applied
by the second variable rotator VPR2 is varied in a continuous or
quasi-continuous manner, with a discretization that follows from
the stepwise incremental nature of the computer-implemented control
scheme. It is convenient that the steps in the phase retardation
.PHI..sub.2 have a constant absolute value , referred to as the
"step angle ", although non-constant step angles, for example
dependent on the absolute value of the phase retardation
.PHI..sub.2, could be used. For example, =.pi./180 radians.
[0113] In general, the smaller the step angle size, the better the
stabilization (smaller uniformity error), but a trade-off with the
stabilization speed need to be considered. In fact, for a given
step angle size , the maximum SOP fluctuation on the Poincare
sphere (see below) in the step time of the control loop that can be
compensated for is about /2.
[0114] The retardation .PHI..sub.2 of VPR2 is varied by the
controller CTRL1 in a predefined range from k.pi. to (k+k').pi.
radians, wherein k is an integer and k' is an odd integer different
from zero. Preferably, k' is equal 1. Such a range may be for
example between 0 and .pi. radians or between .pi. and 2.pi. or
between 2.pi. and 3.pi..
[0115] Whenever the input SOP varies to cause the retardation
.PHI..sub.2 reach a threshold of the predefined range (e.g. k.pi.
or (k+k').pi.), then the retardation .PHI..sub.1 of the first
variable rotator VPR1 is switched by the controller CTRL1 between
the values m.pi. and (m+m').pi. radians, wherein m is an integer
and m' is an odd integer different from zero. Preferably m' is
equal to 1. For example, m may be equal to 0, 1 or 2. At the same
time the sign of the phase retardation increments on the second
variable retarder is reversed. In the normal mode of operation,
when the retardation of VPR2 is not at threshold limit, then the
retardation of VPR1 is kept constant at, e.g., 0 or .pi. radians.
The switching of the retardation of VPR1 allows to overcome the
finite birefringence range wherein VPR2 is operated and to obtain
an endless polarization stabilization, while avoiding any
cumbersome reset procedure. As will be explained below, the
combination of VPR1, WP1 and VPR2 are so that the azimuth value of
the output SOP (SOP.sub.INT) is not appreciably perturbed during
the switching of rotator VPR1, provided that the input SOP
variation is sufficiently small in the switching time.
[0116] At each control period or step the signal control
.phi..sub.2 of VPR2 may be changed so that the respective phase
retardation .PHI..sub.2 changes by a quantity of constant step
angle .theta.. At each step the control algorithm calculates the
error: if the error at the current step becomes larger than the
error at the previous step; then the sign of the retardation
variation is changed, else the sign is not changed. The signal
control .PHI..sub.1 of the phase retardation of VPR1 is kept
constant as long as .PHI..sub.2 is not a limit of the predefined
range, e.g. [0,.pi.]. If the value .PHI..sub.2 has reached a range
limit and the sign of the retardation variation would lead next
step .PHI..sub.2 outside of the range, then the value of
.PHI..sub.2 is not changed at the successive step, whilst the
variation sign is inverted and the value of .PHI..sub.1 is commuted
between 0 and .pi..
[0117] More precisely the control algorithm may consist of the
following exemplary algorithm steps: [0118] 1. assignment of the
constant .alpha., depending on the behavior of the optical
components; [0119] 2. initialization to zero of the error at the
previous step .epsilon..sub.0; [0120] 3. initialization of the
Boolean value S that can assume only the values 0 or 1,
corresponding to the commutation state of the first rotator VPR1;
[0121] 4. initialization of the second rotator retardation
.PHI..sub.2 to the middle range value, e.g. .pi./2; [0122] 5.
initialization of the variation sign .sigma. of the phase
retardation .PHI..sub.2; [0123] 6. initialization of the absolute
value (step angle) of the variation of the phase retardation
.PHI..sub.2; [0124] 7. acquisition of V.sub.1 from the first
photodiode; [0125] 8. (optional in case of V.sub.ref) acquisition
of V.sub.2 from the second photodiode; [0126] 9. calculation of the
current error .epsilon. as absolute value of
(V.sub.1-.alpha.V.sub.2); [0127] 10. if the current error .epsilon.
is greater than the previous error .epsilon..sub.0 then: [0128]
10.1. inversion of the variation sign .sigma.; [0129] 11. variation
of .PHI..sub.2 by a quantity of absolute value .theta. and sign
.sigma.; [0130] 12. if .PHI..sub.2 is not between 0 and .pi. then:
[0131] 12.1. inversion of the variation sign .sigma.; [0132] 12.2.
variation of .PHI..sub.2 by a quantity of absolute value and sign
.sigma.; [0133] 12.3. negation of the Boolean state S, that means
commutation of the state of the first rotator VPR1; [0134] 13.
assignment of the current error .epsilon. to the previous error
.epsilon..sub.0; [0135] 14. updating of .PHI..sub.1 as product
between S and .pi.; [0136] 15. output of the first phase
retardation .PHI..sub.1; [0137] 16. output of the second phase
retardation .PHI..sub.2; [0138] 17. return to algorithm step 7.
[0139] Referring to FIG. 3, the second stage 300 comprises a
polarization transformer PT2 similar to the polarization
transformer PT1 of the first stage 200 described above. Accordingly
it comprises first and second variable rotators VPR3 and VPR4, for
example similar to the variable rotators VPR1 and VPR2 of the first
stage 200, and an interposed quarter-wave plate WP2 oriented at
.+-.45 degree with respect to the defined azimuth. The elements
VPR3, WP2 and VPR4 are arranged along the main beam path `x` of the
polarization stabilizer 100 so as to receive the polarization
multiplexed optical radiation outputting from the polarization
transformer PT1 of the first stage 200. The fully stabilized SOP of
the identified channel outputting from the polarization stabilizer
device 100 is labeled SOP.sub.OUT. The monitoring system MS2 is
identical to the one exemplarily described with reference to FIG.
2. Accordingly, it comprises a polarization insensitive beam
splitter BS2, a fixed linear polarizer P2, a photodiode PD3, which
is apt to generate, via a demodulator DM3, a signal V.sub.3
responsive of the optical power of the extracted polarized portion
of solely the identified channel.
[0140] A controller CTRL2 is connected to the demodulator DM3 and
has first and second outputs connected to the first and second
rotators VPR3 and VPR4 respectively. The signal V.sub.3 is sent to
an input of the electronic controller CTRL2 that generates as
outputs, responsive to the input signal V.sub.3, the control
signals .phi..sub.3 and .phi..sub.4 for setting the rotators VPR3
and VPR4 to the appropriate phase retardation values .PHI..sub.3
and .PHI..sub.4.
[0141] The controller CTRL2 is operable to ensure that the third
variable rotator VPR3 assumes preferably only two retardation
values, e.g. 0 and .pi. radians, while the fourth variable rotator
VPR4 has a retardation step-wise smoothly varying, preferably in
the range from 0 to .pi. radians.
[0142] Separate controllers CTRL1 and CTRL2 are shown for the
polarization stabilizer 100, consistent with their functional
independence from one another. However, it will be understood that
the two controllers could be embodied in a single hardware,
firmware or software unit.
[0143] In operation, the elliptical SOP with fixed axes
(SOP.sub.INT), obtained as output of the first stage 200, is
transformed by the second stage 300 into a fixed linear SOP having
the defined (vertical) azimuth. The operation of the second stage
300 of FIG. 3 is controlled by a feed-back control loop based on
the one described above. The controller CTRL2 executes a control
algorithm similar to the one of the first stage 200, the only
difference being that in step 9 the current error is now the
absolute value of V.sub.3. The aim of the feed-back is to minimize
or maximize (depending on the azimuth orientation of the fixed
polarizer P2) this error.
[0144] It is noted that the use of the Faraday magneto-optic effect
in the polarization stabilizer device 100 allows solving the
problem of the criticality of the orientation of the applied field
and of the optical propagation direction with respect to the
internal structure of the material; a problem which is typically
present in birefringent element based on electro-optic or
acusto-optic effects. In fact, in variable Faraday rotator, the
rotation of the polarization azimuth is directly proportional to
the component of the variable magnetic field applied along the
direction of propagation of the optical radiation. Varying the
direction of propagation and/or the direction of the applied
magnetic field, the resulting eigenstates (i.e. left and right
circularly polarized) do not change.
[0145] FIG. 4 is now referred to explain the principles of
operation of the proposed polarization stabilizer device 100 of
FIG. 3 in terms of a Poincare sphere representation.
[0146] Referring to FIG. 4, each SOP is represented by a point on
the sphere, with longitude 2.eta. and latitude 2.xi.. The angle
.eta. is the azimuth of the major axis of the polarization ellipse
and the quantity tan .epsilon. is the ellipticity with sign plus or
minus according to whether the SOP is left-handed or right-handed.
The poles L and R correspond to the left (.xi.=45.degree.) and the
right (.xi.=-45.degree.) circular SOP respectively. The points on
the equator represent linearly polarized light with different
azimuths .eta.. In particular the points H and V correspond to the
horizontal (.eta.=0.degree.) and the vertical (.eta.=90.degree.)
linear SOP respectively. The points Q and T correspond to the
linear SOP with azimuth .eta.=45.degree. and .eta.=-45.degree.
respectively.
[0147] The action of a fixed polarizer (such as P2 in FIG. 3) is to
transmit only the component of light in a fixed SOP. The
transmitted fraction of the incident optical power is
cos.sup.2(.phi./2), where .phi. is the angle at the center of the
sphere between the representative points of incident and
transmitted SOP.
[0148] For a generic birefringent element there are two orthogonal
states of polarization, said eigenstates, which are not changed by
the element itself. The effect of the propagation through a
birefringent element is represented on the Poincare sphere by a
rotation of an angle .PHI. about a suitable axis. The diametrically
opposite points corresponding to the orthogonal eigenstates belong
to and identify this axis of rotation. The angle of rotation .PHI.
is equal to the phase retardation or phase difference introduced by
the birefringent elements between the eigenstates. In case of
linearly birefringent element, that is an element with linearly
polarized eigenstates, it is possible to define two orthogonal
eigenaxes respectively as the fixed directions of the linearly
polarized optical field corresponding to the eigenstates. A rotator
is represented as a birefringent element having its axis of
rotation on the vertical axis passing through the poles L and R, as
shown in FIG. 4 with the top curved arrow near the symbols
.PHI..sub.1 and .PHI..sub.2 representing the rotation on the sphere
corresponding to the rotators VPR1 and VPR2, respectively.
[0149] In FIG. 4, an arbitrary input SOP (SOP.sub.IN) is first
transformed into SOP.sub.WP1 by the quarter-wave plate WP1, having
its axis of rotation passing through points T and Q and an
associated fixed rotation on the sphere of 90.degree.. Then it is
transformed by the second rotator VPR2 into a SOP (SOP.sub.INT)
represented on the Poincare sphere by a point belonging to the
great circle .GAMMA. including the points L and Q, that is an
elliptical SOP with major axis oriented at .+-.45.degree. with
respect to the (vertical) defined azimuth. Thus, by suitably
controlling the phase retardation .PHI..sub.2 of the second rotator
VPR2 in the exemplary range between 0 and .pi. radians, any input
SOP (SOP.sub.IN) is transformed into a SOP belonging to the great
circle .GAMMA.. In other words, the first stage 200 locks the
polarization state on a meridian of the sphere, i.e. it locks the
polarization azimuth to a defined value represented by a couple of
values mutually orthogonal. It is contemplated that any great
circle on the Poincare sphere may take the place of the meridian
.GAMMA. in FIG. 4, being the locus of the SOPs having one of the
two polarization parameters (or a combination thereof) fixed. The
second stage 300, by controlling the phase retardation .PHI..sub.4,
moves the SOP from the great circle .GAMMA. into the output linear
SOP with azimuth .eta.=90.degree., corresponding to the point V
(trajectory SOP.sub.INT-SOP.sub.WP2-SOP.sub.OUT).
[0150] For the sake of clarity, in FIG. 4 it is assumed that the
first and the third commuted rotators VPR1, VPR3 do not act on the
SOP (.PHI..sub.1=0 and .PHI..sub.3=0).
[0151] The endlessness of the control scheme of the first stage 200
will now be illustrated with reference to FIG. 5. To this purpose,
it will be assumed that the representative point of the input SOP
moves along the exemplary trajectory on the Poincare sphere shown
in FIG. 5a. Four successive representative positions of the input
SOP (labeled with incremental numbers from 1 to 4) will be
considered.
[0152] FIGS. 5b-5e represent the four corresponding SOP evolutions
during the propagation of the optical radiation through the first
stage 200. The points labeled with the subscripts VPR1, WP1 and
VPR2 represent respectively the SOP outputted by the switched
rotator VPR1, the SOP outputted by the linear plate WP1 and the SOP
transmitted by the smoothly varied rotator VPR2.
[0153] Initially (FIG. 5b), the point 1 (SOP.sub.IN) passes
unperturbed the switched retarder VPR1 (phase retardation
.PHI..sub.1=0). Then it is transformed into the point 1.sub.WP1 by
the action of the quarter-wave plate WP1 and subsequently into the
point 1.sub.VPR2 (belonging to .GAMMA.) by the action of the
smoothly varied retarder VPR2 with exemplary phase retardation
.PHI..sub.2=.pi./2.
[0154] The variation of SOP.sub.IN shown in the trajectory from
point 1 to point 2 in FIG. 5a, is compensated by progressively
decreasing the phase retardation .PHI..sub.2 till to zero when the
point SOP.sub.IN intercepts the great circle including V and Q,
i.e. the equator (point 2 in FIG. 5c, .PHI..sub.1=0,
.PHI..sub.2=0). In fact, after the action of WP1, the SOP is
already on the great circle .GAMMA..
[0155] The further variation of SOP.sub.IN according to FIG. 5a
cannot be compensated simply by decreasing .PHI..sub.2 because it
has reached its lower limit. Therefore, in order to obtain an
endless control, the phase retardation .PHI..sub.1 is commuted to
.pi., while .PHI..sub.2 is kept constant (i.e. equal to zero). As
illustrated in FIG. 5d, the polarization azimuth of the input SOP
(point 3) is rotated of .pi./2 by the first variable rotator VPR1
by means of a rotation of .pi. around the vertical axis (i.e.
.PHI..sub.1=.pi., .PHI..sub.2=0). Now the successive variation of
SOP.sub.IN is compensated by increasing .PHI..sub.2 (FIG. 5e,
.PHI..sub.1=.pi., .PHI..sub.2=.pi./2).
[0156] It is important to note that during the commutation of the
first phase retardation .PHI..sub.1 the SOP moves always on the
equator (trajectory 3-3.sub.VPR1 in FIG. 5d), which is subsequently
transformed into the great circle F including L and Q by the
quarter-wave plate WP1. During commutation, the subsequent rotator
VPR2 is either at 0 or .pi., i.e. it transforms the circle .GAMMA.
in itself. In conclusion, during the commutation of VPR1 the SOP
transformed by the first stage 200 remains at the target
polarization azimuth (module 90.degree.), provided that the input
SOP is nearly constant during the commutation.
[0157] The endless operation of the control procedure of the second
stage 300 of FIG. 3 is now described with reference to FIGS. 6a-6c,
under the assumption that the representative point of the incident
SOP (SOP.sub.INT) endlessly moves on the great circle .GAMMA. in
the direction from point Q to point L.
[0158] FIGS. 6a-6c represent the corresponding evolution of the
SOPs during the propagation through the birefringent elements of
the second stage 300 of FIG. 3. The points labeled with the
subscripts VPR3, WP2 and VPR4 represent respectively the SOP
outputted by the switched rotator VPR3, the linear plate WP2 and
the smoothly varied rotator VPR4. In all cases the output SOP is
the linear state represented by the point V.
[0159] Initially (FIG. 6a) the point 1, representative of the first
SOP.sub.INT, is left unaltered by the third rotator VPR3
(.PHI..sub.3=0). Then it is transformed into the point 1.sub.WP2 by
the action of the quarter-wave plate WP2 and subsequently into the
point 1.sub.VPR4 by the action of the smoothly varied rotator with
exemplary phase retardation .PHI..sub.4=3.pi./4. While the
representative point 1 moves along the great circle .GAMMA., the
control algorithm reacts by increasing the phase retardation
.PHI..sub.4 until reaching the value of .pi. when the point
SOP.sub.INT reaches the north pole L, that is to say is left
circularly polarized (FIG. 6b, point 2, .PHI..sub.3=0,
.PHI..sub.4=.pi.). The further variation of SOP.sub.INT can not be
compensated simply by further increasing .PHI..sub.4 because it has
reached the exemplary upper limit of .pi.. Therefore, in order to
obtain an endless control, the phase retardation .PHI..sub.3 is
commuted from 0 to .pi., while .PHI..sub.2 is kept constant, i.e.
equal to .pi. (after commutation: .PHI..sub.3=.pi.,
.PHI..sub.4=.pi.). As illustrated in FIG. 6b, since the point 2
(SOP.sub.INT) is an eigenstate (L) of the variable rotator, it is
not perturbed during the switching of the rotator VPR3. This
assures that the commutation does not perturb the output power,
provided that the SOP.sub.INT is nearly constant during the
commutation. Now the further variation of SOP.sub.INT, as
illustrated in FIG. 6c, is compensated by decreasing .PHI..sub.4
(.PHI..sub.3=.pi., .PHI..sub.4=3.pi./4).
[0160] The proposed endless polarization stabilizer 100 of FIG. 3
for polarization multiplexed system has been experimentally tested.
By varying the electrical current injected in the variable rotators
VPR1, VPR2, VPR3 and VPR4 in the range of about 9/27 mA, it has
been possible to rotate the polarization azimuth in the range
0.degree./90.degree.. The measured response time of the VPRs in
switching the polarization azimuth from 0.degree. to 90.degree. and
vice versa is about 40 .mu.s. This response time is limited by the
electric circuit of the current driver. The control algorithm has
been implemented on a single digital signal processing electronic
controller (CTRL1, CTRL2). The electrical feed-back signals are
generated by the photodiodes (PD1, PD2, PD3) with lowpass-bandwidth
of about 200 kHz, in order not to eliminate the frequency
components around the pilot tone frequency. These spectral
components are needed by the controller for stabilizing the SOP of
the channel identified by the pilot tone. The signals then go
through respective pilot tone demodulator (DM1, DM2, DM3) and are
acquired by the controller, after analog-to-digital conversion.
Three identical pilot tone demodulators are used in the
experimentation, characterized by a 3 dB-bandwidth of about 30 kHz
around the center frequency given by the pilot tone frequency
f.sub.PT=82 kHz. It has been experimentally found that the
demodulator response time is less than 200 .mu.s. Such a response
time is inversely proportional to the 3 dB-bandwidth of the
demodulator. The step time of the digital algorithm implemented on
the controller has been chosen equal to 200 .mu.s in order to
allows each feed-back signal coming to the corresponding pilot tone
demodulator to stabilize.
[0161] At each step of the digital control algorithm the processor
computes the error and generates four control signals. These
signals, after digital-to-analog conversion, control respectively
the current drivers that generate the VPRs input currents.
[0162] The effectiveness of the polarization stabilizer 100 of FIG.
3 in polarization tracking has been first verified by considering a
single 10 Gb/s intensity-modulated channel with on-off-keying
non-return-to-zero (OOK-NRZ) modulation format. To this data
modulation a pilot tone is superimposed as a sinusoidal intensity
modulation at the pilot tone frequency f.sub.PT=82 kHz, with
modulation index m. More precisely the pilot tone modulator adds to
the signal an intensity modulation directly proportional to [1+m
sin(2.pi.f.sub.PTt)]. The measured pilot tone amplitudes
A.sub.i(t)=1 to 3, are directly proportional to m.
[0163] In the experimentation the 10 Gb/s intensity-modulated NRZ
signal is directly detected by a photoreceiver, with electrical
bandwidth of 7.5 GHz, placed after an optical preamplifier. In FIG.
7 the bit-error-rate (BER) as function of the input power (measured
in dBm) to the optical preamplifier is shown for various modulation
index m. The curves 20, 30, 40, 50 correspond to an automatic
polarization tracking driven by a pilot tone with modulation
indexes m respectively equal to 0.025, 0.05, 0.075, 0.10, in
presence of an endlessly varying SOP inputting the stabilizing
device 100. The results are compared with the reference BER curve,
labeled 10, obtained in correspondence of a constant input SOP,
without polarization tracking and without pilot tone. A penalty, at
BER 10.sup.-9, less than 1 dB is measured in case of polarization
tracking and pilot tone with m equal to 0.05. The experimented
penalties are in good agreement with the usual ones suffered by
standard all-optical networks with pilot tones.
[0164] The modulation index m should not exceed a threshold value
in order not to degrade the transmission quality. It should also
not be too low so that the signals V.sub.1, V.sub.2, V.sub.3 have a
sufficiently high signal to noise ratio (electrical noise may be
generated by the photodiode, the demodulator and the controller).
From FIG. 7, a good trade-off range is from 0.01 to 0.10, ends of
range included.
[0165] FIG. 8 shows the result of an assessment of the penalty
generated by the pilot tone and by the polarization tracking for
m=0.05. The curves 10 and 30 are the same as in FIG. 7. The curve
60 corresponds to a pilot tone with m=0.05, an input SOP constant
and no polarization tracking, showing a penalty of about 0.5 dB, at
BER 10.sup.-9, with respect to the case of no polarization tracking
and no pilot tone (curve 10). The BER curve 30 obtained in
correspondence of an endlessly varying SOP and an automatic
polarization tracking driven by a pilot tone with m=0.05 shows a
penalty of less than 0.5 dB with respect to the curve 60. FIG. 8
shows that for m=0.05 the operation of the polarization tracking
gives a very small penalty in addition to the small penalty due to
the pilot tone alone.
[0166] A second alternative embodiment of the polarization
stabilizer of FIG. 2 will now be described with reference to FIG.
9. The same reference numerals will be used for identical
elements.
[0167] The device 100 of FIG. 9 is apt to receive a polarization
multiplexed optical radiation as an input optical radiation having
an identified channel comprising a pilot signal with arbitrary
state of polarization (SOP.sub.IN).
[0168] The polarization multiplexed optical radiation is emitted
from the device 100 as an optical radiation having a stabilized
defined SOP of the identified channel (SOP.sub.OUT). Without loss
of generality, the defined SOP is assumed to be the linear vertical
SOP having the defined azimuth vertical and the defined ellipticity
zero.
[0169] The device 100 comprises a first and a second stage 200 and
300.
[0170] The polarization multiplexed optical radiation traverses the
first stage 200 and outputs the first stage 200 with a SOP of the
identified channel (SOP.sub.INT) having the polarization azimuth
parallel or perpendicular with respect to the defined output
azimuth (i.e. (0.degree.,90.degree.) having assumed a vertical
output azimuth). The optical radiation then traverses the second
stage 300.
[0171] The first polarization transformer PT1 of the first stage
200 comprises a first and a second variable retarder VR1 and VR2.
The second polarization transformer PT2 of the second stage 300
comprises a third and a fourth variable retarder VR3 and VR4. A
variable retarder is a birefringent element having fixed
birefringence eigenaxes and variable controllable phase
retardation. The eigenaxes of the first variable retarder VR1 are
oriented at approximately .+-.45.degree. with respect to the
eigenaxes of the second variable retarder VR2. The same is valid
for the third and the fourth variable retarders VR3 and VR4. The
eigenaxes of the third variable retarder VR3 are oriented
approximately parallel (or perpendicular) with respect to the
eigenaxes of the second variable retarder VR2, and the eigenaxes of
both the variable retarders VR2 and VR3 are parallel (or
perpendicular) with respect to the defined output SOP (vertical
linear).
[0172] The monitoring systems MS1 and MS2 and the controller CTRL1,
CTRL2 of the optical device 100 of FIG. 9 have been described above
with reference to FIG. 2 (or FIG. 2a) and FIG. 3.
[0173] The principle of operation of the optical device 100 of FIG.
9, as well as the control algorithms and the endless mechanism, are
similar to those described with reference to FIG. 3. Further
details are described in patent application WO03/014811 cited
above.
[0174] An third alternative embodiment of the polarization
stabilizer of FIG. 2 will now be described with reference to FIG.
10.
[0175] The device 100 of FIG. 10 is apt to receive a polarization
multiplexed optical radiation as an input optical radiation having
an identified channel comprising a pilot signal with arbitrary
state of polarization (SOP.sub.IN). The polarization multiplexed
optical radiation is emitted from the device 100 as an optical
radiation having a stabilized defined SOP of the identified channel
(SOP.sub.OUT) and an optical power not depending on the input SOP.
Without loss of generality, the defined SOP may be the linear
vertical SOP having the defined azimuth vertical and the defined
ellipticity zero.
[0176] The device 100 comprises a first and a second stage 200 and
300.
[0177] The polarization multiplexed optical radiation traverses the
first stage 200 and outputs the first stage 200 with a SOP
(SOP.sub.INT) having the polarization azimuth at .+-.45.degree.
with respect to the defined output azimuth (i.e.
(-45.degree.,+45.degree.) having assumed a vertical output
azimuth).
[0178] The first polarization transformer PT1 of the first stage
200 comprises a rotating plate RP1 which may be a quarter-wave
plate or a half-wave plate. A rotating plate is a linearly
birefringent element having fixed phase retardation and
birefringence axes with a controllable rotation.
[0179] The second polarization transformer PT2 of the second stage
300 comprises a fixed quarter-wave plate WP2 oriented with its axes
at .+-.45.degree. to the output SOP (vertical) and a rotating
half-wave plate RP2.
[0180] The monitoring systems MS1 and MS2 and the controller CTRL1,
CTRL2 of the optical device 100 of FIG. 10 have been described
above with reference to FIGS. 2, 3 and FIG. 2a.
[0181] The principle of operation of the optical device 100 of FIG.
10 is based on the one described with reference to FIG. 3, provided
that now the endless operation is provided by the infinite rotation
of the plates RP1 and RP2, and the control algorithm needs to be
suitably adjusted in a straightforward way.
[0182] It is noted that the prior art polarization stabilizer
devices based on a two stage scheme, such as for example those
described in WO03/014811, are not suitable to stabilize the
polarization of a polarization multiplexed radiation. In fact, the
superposition of two orthogonally polarized optical channels (e.g.
having the same optical wavelength) results in an overall SOP which
depends on the intensities and the relative phase of the two
channels. With reference to the Poincare sphere representation, in
case of equal intensity of the two orthogonally polarized channels,
the overall SOP is represented by a point P lying on the great
circle equidistant from the two diametrically opposite points
representative of the channels. The actual position of this point P
on the great circle depends upon the relative phase and it moves on
the great circle as the relative phase between the two superposed
channels varies in the range from 0.degree. to 360.degree.. Vice
versa, the overall SOP represented by a point S might be obtained
by superposing two beams with equal intensities and orthogonal SOPs
represented by any couple of diametrically opposite points
belonging to the great circle defined as the maximum circle of
points equidistant from S.
[0183] In an attempt to use the prior art schemes to stabilize a
polarization multiplexed radiation, the Applicant has understood
that those schemes try to stabilize the overall SOP and not the
orthogonal SOPs of each channel. In fact, when the overall SOP is
stabilized, for example, with reference to FIGS. 3 and 4, in the
vertical linear point V, the orthogonal SOPs of the two channels
lie on the great circle .GAMMA. of FIG. 4 and they move along
.GAMMA. as their relative phase changes.
[0184] The Applicant has also understood that identifying and
measuring the identified channel at the output end of solely the
second stage would not provide the desired technical effects. In
fact, the first stage 200 would lock the overall SOP, with
exemplary reference to FIGS. 3 and 4, on the meridian .GAMMA. while
the two SOPs of the channels would rapidly fluctuate everywhere on
the Poincare sphere. The second stage 300 would not be able to lock
the SOP of the identified channel in the point V, as it is designed
to transform a generic point on .GAMMA. in V and the SOP of the
identified channel does not lie on .GAMMA.. Having recognized these
problems, the Applicant has understood that a suitable design of
the stabilizer device 100 of the present invention would have
provided the desired performances.
[0185] It will be appreciated that the polarization stabilizer
device 100 of the present invention provides an output optical
radiation having a fixed linear SOP of the identified channel.
However, other devices based on this design could provide any other
defined SOP that may be desired. For example, circularly polarized
SOP, or elliptically polarized SOP, or linearly polarized SOP with
a time variant rotation of a desired angular velocity. To generate
a fixed elliptical output SOP, instead of a linear output SOP, it
is sufficient to produce a fixed linear SOP as described above and
then obtain an elliptical SOP with a half-wave plate followed by a
quarter-wave plate, both fixed and suitably oriented. Another
alternative is to add a rotating half-wave plate to transform a
fixed linear SOP into a rotating linear SOP. The polarization
stabilizer devices 100 of FIG. 3 can also be modified to obtain any
fixed output linear SOP other than vertical linear SOP by suitable
rotation of the element WP1 and WP2 (rotation of the eigenaxes
azimuth) and the elements PBS and P2. This generalized
configuration is obtained from the configuration represented in
FIG. 4 by a suitable rotation of the Poincare sphere about the
vertical (L-R) axis.
[0186] More in general, any rigid rotation of the Poincare sphere
shown in FIG. 4 results in a respective configuration of the
polarization stabilizer device 100 shown in FIG. 3 which is
contemplated by the present invention. The same reasoning hold for
devices 100 of FIGS. 9 and 10.
* * * * *